Scintillators are substances which, when hit by radiation such as alpha rays, beta rays, gamma rays, X rays, or neutron, absorb the radiation to emit fluorescence. The scintillator is combined with a photodetector, such as a photomultiplier tube, and such a combination is used for detection of radiation. The combination has wide varieties of application fields, including medical fields such as tomography, industrial fields such as nondestructive inspection, security fields such as inspection of personal belongings, and academic fields such as high energy physics.
Among scintillators are various types of scintillators according to the type of radiation and the purpose of use. Concrete examples include inorganic crystals such as bismuth germanium oxide (Bi4Ge3O12) and cerium-containing gadolinium silicon oxide (Gd2SiO5:Ce), organic crystals such as anthracene, polymers such as polystyrene or polyvinyltoluene incorporating an organic fluorescent substance, liquid scintillators, and gaseous scintillators.
Conventional neutron detection has mainly employed a neutron detector using a 3He gas. Owing to price hikes of a rare 3He gas, however, switching to alternative technologies is desired. A neutron detector using a solid scintillator for detecting neutrons is one of promising candidates for the alternative technologies.
As typical properties demanded of a scintillator, a high amount of luminescence, high stopping power for radiation, fast decay of fluorescence, etc. are named. Particularly in the scintillator whose object of detection is neutron, a radiative capture reaction occurs between neutrons and an absorbing substance contained in materials for the detector or the object to be tested, such as Fe (iron), Pb (lead), Cd (cadmium), C (carbon) or N (nitrogen). As a result, gamma rays are prone to occur, so that the scintillator needs to have the ability to discriminate between gamma rays and the neutron.
The solid scintillator so far used for neutron detection has been a 6Li glass scintillator as a material lacking deliquescent properties and making a rapid response. However, its manufacturing process has been complicated and thus expensive, and there has been a limit to its upsizing. A scintillator for neutron detection, comprising a fluoride crystal, on the other hand, is advantageous in that an upsized scintillator can be produced at a low cost. For example, a scintillator comprising a LiBaF3 crystal has been proposed. However, this scintillator has high sensitivity to γ rays, and produces a great background noise attributed to γ rays. Thus, there has been need to take a special measure in using it as a scintillator for neutron detection (see Non-Patent Document 1).
In the light of the above problems, the inventors of the present invention irradiated various fluoride crystals with neutron, and evaluated their properties as scintillators for neutron detection. As a result, they found that an LiCaAlF6 crystal containing a lanthanoid element exhibited particularly satisfactory characteristics as a scintillator for neutron detection by incorporating therein 1.1 to 20 atoms of 6Li per unit volume (nm3) (see Patent Document 1).
The LiCaAlF6 crystal containing the lanthanoid element has high detection efficiency for neutron and has high ability to discriminate between neutron and gamma rays. For use as a solid scintillator for neutron detection, intended for replacing the aforementioned neutron detector using a 3He gas, however, this LiCaAlF6 crystal has left room for improvement in the discrimination ability. Regrettably, there have been no generally accepted theories for predicting the discrimination ability. Hence, it has been difficult to predict beforehand whether or not various materials have the ability to discriminate between neutron and gamma rays.